chapter 4 transient stability margin of scig in...

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CHAPTER 4

TRANSIENT STABILITY MARGIN OF SCIG IN WIND

FARM USING STATCOM

4.1 INTRODUCTION

Angular stability assessment of WEG is one of the main issues in

power system security and operation. Rotor speed stability refers to the ability

of an induction machine to remain connected to the electric power system and

running at a mechanical speed close to the speed corresponding to the actual

system frequency after being subjected to a disturbance (Kanabar 2008). In

practice, overspeed protection circuit disconnects the WEG from the grid

when its speed exceeds 1.2pu. From the power quality study undertaken in

one 110kV/11kV substation at Anthiyur windfarm it was observed that nearly

60% of power quality issues in windfarms are contributed by voltage

sags,29% by voltage swells,8% by transients and 3% by interruptions

(Thirumoorthy 2009). Normally, LVRT requirements are stringent in regions

with high penetration of wind power. In order to promote the integration of

wind farms into the electrical network, FACTS are widely used. STATCOM

is one of them (Hingorani 2000). STATCOM stimulates voltage stability by

reactive power regulation. STATCOM provides or absorbs reactive power to

or from the grid to compensate small voltage variations at PCC. Many studies

show that STATCOM helps the wind farm to stabilize voltage especially after

a voltage dip occurs. With regard to maintaining the short term voltage

66

stability, all grid codes demand that the voltage in the transmission power grid

is re-established without subsequent disconnection of large wind farms.

In this chapter, the effect of STATCOM on the transient stability

margin of SCIG is studied under different penetration levels in the event of

unbalanced or balanced fault in the grid. The performance of WEG with

STATCOM is studied using MATLAB/Simulink taking into account the

nature of the load and the results are presented.

4.2 WIND FARM STABILITY AND REACTIVE POWER

COMPENSATION

A system experiences a state of voltage instability when there is a

progressive or uncontrollable drop in voltage magnitude after a disturbance,

increase in load demand or change in operating condition. The main factor,

which causes these unacceptable voltage profiles, is the inability of the

distribution system to meet the demand for reactive power (Alejandro Jurado

2009). The reactive power absorbed by the induction generator coupled to

wind turbine depends on the generator parameters and its operational points

(generated electric power, terminal voltage magnitude and slip). During the

fault, the generator speed is increased by the difference between

electromagnetic torque of SCIG and mechanical torque of WT. Once the fault

is cleared, the SCIG draws a large amount of reactive power from the grid

because of its high rotational speed. If the rotor accelerates faster than the

terminal voltage is restored, the reactive power consumption continues to

increase. This leads to a decrease in the terminal voltage and thus to a further

deterioration of the balance between mechanical and electrical power and to a

further acceleration of the rotor. Owing to this reactive power consumption, it

can happen that the terminal voltage recovers only relatively slowly after the

67

fault is cleared. This decline in the electromagnetic torque causes a decrease

in the value of the CCT and hence the transient stability margin of SCIG.

4.3 EFFECT OF ADDITIONAL REACTIVE POWER SUPPORT

ON TRANSIENT STABILITY MARGIN OF SCIG

Figure 4.1 shows the torque-slip characteristics of a SCIG with two

different values of reactive power compensation (Kanabar 2008). For a given

set of machine parameters, the electromagnetic torque developed by the WEG

depends on the value of reactive power compensation .The additional value of

reactive power compensation will shift the torque-slip characteristic of SCIG

upwards. Consequently, the value of critical clearing slip will increase from

Scr1 to Scr2. which will enhance the rotor stability margin of SCIG. This, in

turn improves the CCT, which is in compliance with the LVRT requirements

in new grid code.

Figure 4.1 Torque-slip characteristic of SCIG with nominal and

additional reactive power

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4.4 ADVANTAGES OF STATCOM

Due to the low cost, shunt capacitors are the most commonly used

scheme to compensate reactive power in WEGs. Shunt capacitors are used in

banks and switched in and out of the circuit using contactors. Due to the surge

current taken by the capacitors while switching in, the lifetime of the

contactors is limited. The switching of capacitors excites transients and the

switching has to be done by keeping the transients minimum. Also the voltage

support provided will be discontinuous. STATCOM has better characteristics

than FC compensation and SVC. Reactive power output of STATCOM is

independent of the actual voltage at PCC. In contrast, the reactive output of

FC and SVC is proportional to the square of the voltage magnitude at PCC.

This makes the reactive power output from SVC to decrease rapidly when the

voltage at PCC decreases, thus reducing the system stability.

Nevertheless, FACTS systems provide faster and smoother

response to changes in wind farm voltage. On the other hand, shunt capacitors

give a poor response. Power quality issues in Anthiyur windfarm near

Udumalpet in Tamilnadu, show frequent failure of lightning arrestors and

studies show that switching out of capacitor may be one of the reasons which

would have caused the transients that leads to the failure of insulation.

4.5 STATCOM

4.5.1 Principle of Operation

During the last few decades, development of power electronics

technology has helped to propose and implement FACTS devices for

overcoming power quality problems in power system. A STATCOM is a

regulating device used on alternating current electricity transmission

networks. It can act as either as a source or sink of reactive power to an

69

electricity network. When system voltage is low, the STATCOM generates

reactive power (STATCOM capacitive). When system voltage is high, it

absorbs reactive power (STATCOM inductive).The variation of reactive

power is performed by means of a VSC connected on the secondary side of

a coupling transformer. The VSC uses forced-commutated power electronic

devices GTOs, IGBTs or IGCTs which can be operated at high switching

frequency to synthesize a voltage from a DC voltage source. The

STATCOM can be operated in two different modes:

In voltage regulation mode (the voltage is regulated within

limits)

In VAR control mode (the STATCOM reactive power output is

kept constant)

When the STATCOM is operated in voltage regulation mode, it

implements the V-I characteristic shown in Figure 4.2.

Figure 4.2 STATCOM V-I characteristic

As long as the reactive current stays within the minimum and

minimum current values (-Imax, Imax) imposed by the converter rating, the

voltage is regulated at the reference voltage Vref. However, a voltage droop is

normally used (usually between 1% and 4% at maximum reactive power

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output), and the V-I characteristic has the slope indicated as shown in the

Figure 4.2.

4.5.2 Mathematical Model of STATCOM Control System

The STATCOM used is a standard 3-phase inverter with PWM

switching. The passive elements, namely, the series choke and the dc-bus

capacitor are designed to limit the ripple in the ac side current and dc bus

voltage of the STATCOM, respectively.

Figure 4.3 Schematic diagram of STATCOM connected to the grid

Figure 4.3 shows the schematic diagram of STATCOM connected

to grid. Assuming that the control of STATCOM is successful, the current

that will flow through R and L is equal to the reference current Ii .The voltage

that the inverter should generate is given below by applying Kirchhoff’s

voltage law. R is the equivalent loss resistance which includes winding

resistance, switch power loss etc. L is the filter inductance, Vg is the grid

voltage and Vi is the inverter output voltage before filtering (Arun

Karuppusamy 2007). Since the current references in the Synchronous

Reference Frame strategy are in the d-q plane, the equations are first written

in the R-Y-B plane and then they are transformed to the plane and

subsequently to the d-q plane. Applying KVL to the R-L circuit shown in

Figure 4.3, the Equations (4.1) to (4.3) are obtained:

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iaia ga ia

di (t)(t) (t) R.i (t) L.

dt(4.1)

ibib gb ib

di (t)(t) (t) R.i (t) L.

dt(4.2)

icic gc ic

di (t)(t) (t) R.i (t) L.

dt(4.3)

Converting the above equations to plane, Equation (4.4) to (4.5) are

obtained.

ii g i

di (t)(t) (t) R.i (t) L.

dt (4.4)

i

i g i

di (t)(t) (t) R.i (t) L.

dt(4.5)

In general, Equation (4.6) can be written for STATCOM.

ii g i

dIV V R.I L

dt(4.6)

As it is known that plane is related to d-q plane by the relation

given by Equation (4.7), Equation (4.8) is obtained.

( + j ) = (d cos - q sin ) + j ( d sin + q cos ) = (d + jq).e j

(4.7)

j

id iqj j j

id iq id iq gd gq

d i ji .ej .e R i ji .e L j .e

dt(4.8)

The above equation , when multiplied by e-j is transformed to d-q

plane. Since d-axis is aligned with grid voltage, Equation (4.9) to (4.10) are

obtained.

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idid id iq

diR.i L L.i | V |

dt(4.9)

iq

iq iq id

diR.i L L.i

dt (4.10)

From the above two Equations, the d axis and q axis currents Iid and

Iiq can be represented as shown in Figure 4.4.

Figure 4.4 Representation for d axis and q-axis currents of STATCOM

Where

id id iq' Li | V | (4.9)

iq iq id' Li (4.10)

Above mathematical equations can be represented as block diagram

shown in Figure 4.5, in which the d-axis and q-axis reference voltage vid and

viq of STATCOM are obtained. Reactive power control is achieved by control

of Iiq and active power control by control of Iid .

73

Figure 4.5 Representation of d-axis and q-axis voltage at PCC

Similarly for DC bus voltage controller, Equation (4.11) can be

obtained.

Iid = C dVdc/dt + Vdc/R (4.11)

Figure 4.6 shows the representation for DC bus voltage controller

of STATCOM.

Figure 4.6 Representation of STATCOM DC bus voltage controller

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4.5.3 Design and Control of STATCOM

The design of a STATCOM has three broad sections (Arun

Karuppaswamy 2007):

1. Current reference generation (It involves computing the

reactive current absorbed by SCIG).

2. Design of the DC bus capacitor and inductor

3. Design of closed loop controller, that makes the STATCOM

current to follow the reference.

The first part of the design is to generate the current reference.

There are several methods to generate the current reference. The present study

is based on the application of co-ordinate transformations to separate the

active and reactive components of the current. The strategy used is the Vector

control method (Arun Karuppaswamy 2007). Once the current reference has

been generated, the next work is to find the values of DC capacitor and

inductor of STATCOM, according to the requirement of the reactive power

compensation.

The reactive current injected is controlled so as to obtain full rated

grid voltage before, during and after the fault. It is based on the measurement

of voltage at PCC. The voltage error signal is obtained by comparing the

actual and reference voltage, which is fed to a PI controller. There needs to be

another voltage controller to maintain a constant DC bus voltage. The

STATCOM current is continuously compared with reference current received

from two voltage controllers and error signal is fed into the Hysteresis

comparator.

75

Hysteresis current control is a method of controlling a voltage

source inverter so that an output current is generated which follows a

reference current waveform. This method controls the switches in an inverter

asynchronously to ramp the current through an inductor up and down so that

it tracks a reference current signal. This scheme is employed for generation of

pulses to the STATCOM. This is a continuous current variable switching

current control scheme. The STATCOM current is continuously compared

with the reference current waveform and the error signal after amplification is

fed into the hysteresis comparator. The comparator changes state when the

error exceeds a preset value in positive and negative directions. The

comparator state switches is used to decide which of the switches should be

on and which of the switches should be off. When the STATCOM current

actually goes above the reference current by the comparator hysteresis band,

the comparator changes state. This state change is used to switch off the boost

switch and current ramps down. When the STATCOM current goes below the

reference current by comparator hysteresis band, it changes state again and

state change is used to turn the boost switch on. Thus the STATCOM current

is always maintained within half of the hysteresis band. A hysteresis current

controller is implemented with a closed loop control system and is shown in

diagrammatic form in Figure 4.7(a) (David 2009). An error signal, e(t), is

used to control the switches in an inverter. This error is the difference

between the desired current, iref(t), and the current being injected by the

inverter, iactual(t). When the error reaches an upper limit, the IGBTs are

switched to force the current down. When the error reaches a lower limit the

current is forced to increase. The minimum and maximum values of the error

signal are emin and emax respectively. The range of the error signal, emax –

emin, directly controls the amount of ripple in the output current from the

inverter and this is called the Hysteresis Band. The hysteresis limits, emin and

emax, relate directly to an offset from the reference signal and are referred to

as the Lower Hysteresis Limit and the Upper Hysteresis Limit. The current is

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forced to stay within these limits even while the reference current is changing.

The ramping of the current between the two limits is illustrated in Fig 4.7(b).

Figure 4.7 Block diagram and operational waveform of Hysteresis

current controller

Figure 4.8 shows the total control block diagram of the vector

control scheme for STATCOM.

Figure 4.8 Block diagram of the Vector Control Scheme for STATCOM

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4.6 SIMULATION RESULTS

One of the methods to meet the LVRT requirements is by providing

additional reactive power support which can improve the terminal voltage

during a disturbance. For the purpose of analysis, the system shown in Figure

2.7 is considered with VAR compensation as STATCOM. The SCIG acts as a

load requiring variable reactive power. It was found that SCIG is drawing a

reactive power of nearly 900kVAR during severe three phase to ground fault

with no VAR compensation. The STATCOM’s power rating is to be decided

based on the reactive power requirement. It is discussed in section 4.3, that

additional reactive power improves the transient stability margin of SCIG. A

STATCOM of 1000kVAR is assumed to be installed at PCC as SCIG is

drawing approximately 900kVAR during severe three phase to ground fault

without any compensation. Simulation studies have been carried out assuming

that the system is operating at full load and 12m/s wind speed. Different types

of faults are simulated at PCC. Simulations are repeated for the system with

1000kVAR FC compensation.

Table 4.1 shows the maximum reactive power (Q) consumption of

WEG, maximum SCIG speed and settling time after the fault for different

fault conditions with FC compensation and STATCOM compensation. The

slip of SCIG after the fault clearance is larger than that prior to the fault. The

larger the slip, the larger will be the reactive power demand of SCIG. Results

show that most of the parameters are reduced when 1000 kVAR STATCOM

is used for compensation instead of 1000 kVAR FC, which means that

STATCOM is responding faster than FC.

78

Table 4.1 Comparison of FC compensation with STATCOM

compensation for different fault conditions at the wind speed of

12m/ s

FC compensation STATCOM compensationNature of

fault and

fault

duration

Maximum

Q (kVAR)

Maximum

SCIG

speed

(rad/s)

Settling

time

after the

fault (s)

Maximum

Q(kVAR)

Maximum

SCIG

speed

(rad/s)

Settling

time

after the

fault (s)

Single line to

ground fault

(600ms)

500 168 1.5 400 167 1.25

Double line

to ground

fault(100ms)

1080 175 1.25 920 178 0.8

Three phase

to ground

fault(50ms)

925 186 1 620 179 0.9

For considering the effect of wind penetration level on transient

stability of SCIG, the two machine system shown in Figure 3.6 is taken for

study with VAR compensation as STATCOM. Assuming that the system

under consideration is operating at full load, the transient stability of SCIG

under different fault conditions of various fault durations with STATCOM

compensation is studied.

79

4.6.1 250 kW SCIG Connected to 2000kVA Alternator (Medium

Penetration)-Case 1

4.6.1.1 Single line to ground fault

A single line to ground fault is simulated at the instant of 2 seconds

from the start at PCC. The fault is cleared after 100ms. The wind speed is

assumed to be 10m/s, which is the normal case prevailing in practice.

Simulation is repeated for different fault durations and corresponding values

of the performance indices are given in Table 4.2. It is observed that for

longer duration faults, dip in DC link capacitor voltage is more. STATCOM

DC link voltage Vdc is maintained at 600V before and after fault. Alternator

speed and Vpcc settle at 1 pu.

Figure 4.9 shows the plots of the parameters for a fault duration of

625ms. From Figure 4.9 , it is inferred that grid code is satisfied for single line

to ground fault as the system returns to stable condition without getting

tripped for 625ms fault duration.

Table 4.2 Range of transients in different parameters at SCIG terminals

for single line to ground fault at PCC(case 1)

Fault

Duration(ms) ( rad/sec) P(kW) Q(kVAR) Te(Nm) Vpcc(pu) Vdc(V)

100 157.5-160.5 120-200 79-109 265-1800 0.98-1 576-643

625 156.6-160.8 119-200 75-109 265-1800 0.98-1 570-643

80

Figure 4.9 System performance indices for single line to ground fault of

625ms duration at PCC for a wind speed of 10m/s at full

load of 0.9 power factor lagging (case 1)

81

4.6.1.2 Double line to Ground fault

A double line to ground fault is implemented at PCC. Table 4.3

shows the results for double line to ground fault of different durations. During

the fault, the alternator speed varies over 0.99 to 1.03pu. For 100ms fault

duration, Vpcc and Vdc settle at 0.985pu and 600V respectively. For 400ms

fault, Vpcc and Vdc settle at respective values of 0.945 pu and 580V .For

500ms fault duration, Vpcc and Vdc settle at 0.94 pu and 570V respectively.

Figure 4.10 shows the plots for 100ms fault duration.


for double line to ground fault at PCC for a wind speed of

10 m/s(case 1)

Fault

duration(ms) ( rad/sec) P (kW)

Q

(kVAR)Te (Nm)

Vpcc

(pu)Vdc (V)

100 151.4-171-27 to

+330

-360 to

+590

+4370 to

-7075

0.39

to1.08390-917

200 151.4-171-28 to

+250

-510 to

+605

+4370 to

-7075

0.38 to

1.045400-910

400 151.5-188.5-27.5 to

308

-700 to

+625

+4370 to

-7075

0.36

to0.96400-805

500 151.5-202-55 to

210

-640 to

+600

+4370 to

-70750.36- 0.93 300-845

When the fault duration is increased to 550ms, SCIG speed

increases indefinitely and the system becomes unstable. Vpcc dips to 0.355pu

during fault and settles at 0.917pu after the fault. Figure 4.11(i) shows the

plots of and Te for 550ms double line to ground fault at PCC for a wind

speed of 10m/s at full load. But when the load demand is reduced to half, the

system retains its stability by returning to original condition. Alternator speed

settles at 1.017 pu. Vpcc, SCIG speed and Te respectively settle at 0.92pu,

171rad/s and 975Nm.

82

Figure 4.10 System performance indices for double line to ground fault

of 100ms duration at PCC for a wind speed of 10m/s at full


83

At half load, even though the operating speed of SCIG is high, the

transient stability margin of SCIG is better than that with full load. This is

because of the fast response of STATCOM. Table 4.4 shows the parameter’s

variations for half load .Figure 4.11(ii) shows the plots of and Te

corresponding to this condition.

Figure 4.11(i) and Te for double line to ground fault of 550ms duration

at PCC for a wind speed of 10m/s at full load of 0.9 power

factor lagging (case 1)


for 550 ms double line to ground fault at PCC for a wind speed

of 10 m/s at half load of 0.9 power factor lagging (case 1)

Fault duration

(ms) (rad/s) P(kW) Q(kVAR) Te(Nm) Vpcc(pu) Vdc(V)

550159.6-

182

-25 to

+260

-550 to

+620

+5455 to -

87000.39-1 400-965

84

Figure 4.11(ii) and Te for double line to ground fault of 550ms duration

at PCC for a wind speed of 10m/s at half load of 0.9 power


Figure 4.11(iii) and Te or double line to ground fault of 550ms duration



When the wind speed is reduced to 8m/s from 10m/s for 550ms

fault at full load, the system regains to original condition and the system

becomes stable. Table 4.5 shows the variations for 8m/s during fault

85

condition. Figure 4.11(iii) shows the plots of and Te. Vdc, P,Q and Te

come to original values after the fault clearance.


for 550ms double line to ground fault at PCC for a wind speed

of 8 m/s at full load of 0.9 power factor lagging (case 1)

Fault duration

(ms) (rad/s) P(kW) Q(kVAR) Te(Nm) Vpcc(V) Vdc(V)

550145.8-

167.8

+145 to -

132

-450 to

+600

- 8050 to

+5370

0.39 to

1.065415-950

When the fault duration is increased to 625ms at half load of 0.9

power factor lagging, the system still returns to stable condition after the

clearance of the fault. SCIG speed, P, Vdc and Te respectively settle at

171rad/s,160kW,600V and 950Nm.Alternator speed settles at 1.01pu in 8s.

Vpcc settles at 0.905pu.Table 4.6 shows the transients during fault. Figure

4.12 shows the plots of and Te for this condition.

Figure 4.12 and Te for double line to ground fault of 625ms duration

at PCC for a wind speed of 10m/s at half load of 0.9 power


86


for 625ms double line to ground fault at PCC for a wind speed

of 10 m/s at half load of 0.9 power factor lagging (case 1)

Fault duration


625165.8-

187

-25 to

270

-550 to

+620

3120 to -

53800.39-0.995 385-885

4.6.1.3 Three phase to Ground fault

A three phase to ground fault at the generator terminals is

considered for the study. Table 4.7 shows the different parameter variations

and system becomes stable after the clearance of the fault. Alternator speed

varies over 0.96 to 1.055pu during fault. For 50ms, 100ms, 200ms and 250ms

fault durations, Vpcc settle at 1pu, 0.955 pu, 0.94 pu and 0.92 pu

respectively. Figure 4.13 shows the plots for 100ms fault.


for three phase to ground fault at PCC for a wind speed of 10

m/s at full load of 0.9 power factor lagging (case 1)

Fault duration


50143.5-

179

-255 to

+410

-370 to

+200

+2700 to -

80000-1.075 350-835

100143.5-

180

-110 to

+355

-440 to

+200

+2700 to -

80250-1.075 288-868

200143.5-

203

-88 to

+253

-575 to

+200

+2700 to -

80300-0.985

240-

1310

250143.5-

211.5

-72 to

+222

-620 to

+200

+2700 to -

81000-0.865

200-

1545

87

Figure 4.13 System performance indices for three phase to ground fault



88

When the fault duration is increased to 280ms duration, the system

becomes unstable for RL load of 0.9 power factor lagging. Figure 4.14(i)

shows the plots of Te and SCIG speed for this condition. If unity power factor

load is used for the same type and duration of fault and 10m/s wind speed, the

system retains its original condition and thereby stability is attained. Table 4.8

shows the variations for this condition. Figure 4.14(ii) shows the plots of Te

and SCIG speed.

Figure 4.14(i) and Te for three phase to ground fault of 280ms

duration at PCC for a wind speed of 10m/s at full load of

0.9 power factor lagging (case 1)

Table 4.8 Range of transients in different parameters at SCIG

terminals for three phase to ground fault at PCC for a wind

speed of 10 m/s at full load unity power factor (case 1)

Fault

duration

(ms)(rad/s)

P(kW) Q(kVAR) Te(Nm) Vpcc(pu) Vdc(V)

280215-

139

-79 to

+225

-640 to

+220

+2790 to -

85200-1.06

220-

1555

When the fault duration is increased to 300ms, the system becomes

unstable. For same type of fault and duration, when the wind speed is reduced

89

from 10m/s to 8m/s, the system remains stable. Table 4.9 shows the variations

during fault. After fault, Vpcc and SCIG speed settle at 0.92pu and 157rad/s

respectively.

Figure 4.14(ii) and Te for three phase to ground fault of 280ms


unity power factor (case 1)



speed of 8 m/s at full load of 0.9 power factor lagging(case 1)

Fault

duration

(ms)(rad/s)


300139-

175.5

-92 to

+230

-600 to

+200

+2600 to

-81800-1.025 240-860

4.6.2 250 kW SCIG Connected to 910kVA Alternator (High

Penetration)-Case 2

The penetration level of WEG is increased to 27% by connecting

the 250 kW SCIG to 910kVA alternator. Assuming that the load demand is

high, the simulation is carried out for different fault conditions.

90

4.6.2.1 Single line to ground fault

A single line to ground fault is simulated at PCC.

Figure 4.15 System performance indices for single line to ground fault of

100ms duration at PCC for a wind speed of 10m/s at full

load of 0.9 power factor lagging(case 2)

91

Table 4.10 shows the variations for different fault durations.

Figure 4.15 shows the plots for 100ms fault. When compared to medium

penetration level, the range of transients is increasing for same type and

duration of fault. Vpcc settles at 1pu, 0.98pu and 0.975pu for fault durations

of 100ms, 400ms and 625ms respectively. Alternator speed settles at 1 pu for

all cases.


terminals for single line to ground fault at PCC for a wind

speed of 10 m/s at full load of 0.9 power factor lagging (case 2)

Fault

duration

(ms)

(rad/s) P(kW) Q(kVAR) Te(Nm) Vpcc(pu) Vdc(V)

100 156.4-161.2 113-230 33-108 100-2100 0.955-1.035 524-671

400 155.8-161.2 113-230 45-109 100-2100 0.955-1.02 510-665

625 155-161 100-230 45-109 100-2100 0.955-1.01 525-665

4.6.2.2 Double line to ground fault

A double line to ground fault is implemented at PCC. Table 4.11

shows the parameter variations for double line to ground fault of different

durations. Figure 4.16 shows the plots for 100ms fault. Vpcc settle at 0.955pu,

0.95pu and 0.945pu respectively for 100ms, 200ms and 250ms faults. All

other parameters return to pre fault values.


terminals for double line to ground fault at PCC for a wind


Fault

duration

(ms)(rad/s)


100147-175.5

-58 to305

-300 to+525

+4750 to -8300

0.375-1.07 285-1080

200147-193.5

-30 to255

-440 to+525

+4750 to -8330

0.33-1.03 310-893

250 147-201-30 to220

-450 to+520

+4750 to -8330

0.32-0.99 250-820

92

Figure 4.16 System performance indices for double line to ground fault



93

When the fault duration is increased to 300ms, the SCIG speed

increases indefinitely and becomes unstable. Figure 4.17(i) shows the plots of

Te and SCIG speed.. But, for the same fault duration, when the wind speed is

reduced to 8m/s from 10m/s, the system becomes stable. Table 4.12 shows the

results and Figure 4.17(ii) shows the plots of Te and SCIG speed. It shows

that lesser wind speed increases the transient stability margin of SCIG. Vpcc,

Vdc, SCIG speed and Te settle at 0.95pu, 600V, 157 rad/s and 450Nm

respectively.

Figure 4.17(i) and Te for double line to ground fault of 300ms duration




terminals for double line to ground fault at PCC for a wind


Fault duration


300148.5-

166.6

-85 to

+145

+560 to

-400

+3410 to

-5380

0.365-

1.075

370-

1115

94

Figure 4.17(ii) and Te for double line to ground fault of 300ms duration


factor lagging(case 2)

4.6.2.3 Three phase to ground fault

A three phase fault is simulated at PCC. Variations in different

parameters during fault condition are given in the Table 4.13. Alternator

speed varies over 0.96 to 1.06pu for all faults. For 50ms, 100ms and 150ms,

Vpcc settle at 0.93pu, 0.915pu and 0.88pu respectively. Figure 4.18 shows the

plots for 50ms fault.




Fault

duration

(ms)

(rad/s) P(kW) Q(kVAR) Te(Nm) Vpcc(pu) Vdc(V)

50 145.5-178-165 to

+340

-295 to

+165

+2400 to

-72700-1.038 255-980

100 145.5-182-45 to

250

-360 to

+165

+2400 to

-72700-1.025 180-1050

150 145.5-193-35 to

240

-420 to

+165

+2400 to

-72700-0.93 60-1350

95

Figure 4.18 System performance indices for three phase to ground fault



96

Figure 4.19(i) and Te for three phase to ground fault of 200ms



When the fault duration is increased to 200ms, the system becomes

unstable at full load. Figure 4.19(i) shows the plots of Te and SCIG speed.

But at half load, for same fault, the system regains to original condition. It

shows that STATCOM is fast in producing counter balancing electromagnetic

torque of SCIG. Vpcc, SCIG speed, P,Q, Vdc and Te respectively settle at

0.92pu,173 rad/s, 160kW, 85kVAR, 560V and 950Nm. Table 4.14 shows the

results. Alternator speed settles at 1.01 pu in 9s. Figure 4.19(ii) show the plots

of Te and SCIG speed.

97

Figure 4.19(ii) and Te for three phase to ground fault of 200ms

duration at PCC for a wind speed of 10m/s at half load of




speed of 10 m/s at half load of 0.9 power factor lagging (case 2)

Fault

duration

(ms)(rad/s)


200159-

213

-45 to

230

-500 to

+200

2725 to -

72000-0.935 35-1545

4.7 SUMMARY

This chapter analyzed the impact of penetration level and load

demand on the transient stability margin of SCIG coupled with WT in the

event of any unbalanced or balanced fault in the grid. It is seen that the

reactive power consumption of SCIG during fault is reduced when 1000

kVAR STATCOM is used for compensation instead of 1000 kVAR FC. Table

4.15 and Table 4.16 give the summary of the transient stability margin(in ms)

98

of SCIG for a wind speed of 10m/s at different loading conditions for medium

and high penetration levels respectively.

Table 4.15 Transient stability margin(in ms) of SCIG for a wind

speed of 10m/s at different loading conditions for medium

penetration

Fraction of LoadType of fault

Full load Half load

Nature of load RL load R load RL load R load

Single line to ground fault 625 625 625 625

Double line to ground fault 500 540 625 625

Three phase to ground fault 270 320 320 360

Table 4.16 Transient stability margin(in ms) of SCIG for a wind speed

of 10m/s at different loading conditions for high penetration

Fraction of LoadType of fault

Full load Half load

Nature of load RL load R load RL load R load

Single line to ground fault 625 625 625 625

Double line to ground fault 250 300 370 400

Three phase to ground fault 170 230 220 270

From Table 4.15 and Table 4.16, it can be seen that at half load, the

transient stability margin of SCIG is better than that with full load even

though the operating speed of SCIG is high. This is because of the fast

response of STATCOM. For highly resistive load, the transient stability

margin is increasing, as the resistance component of load offers damping

effect to rotor acceleration.

chapter 4 transient stability margin of scig in...

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